2.1. Wound Healing
A wound can be considered a physical interruption in the normal architecture of tissues, which can result from physical or chemical causes, such as burns, abrasions, cuts and surgical procedures. In the case of skin, since it normally functions as a first line of defense, a cutaneous wound may severely compromise an individual's ability to resist infectious agents, rendering the individual susceptible to opportunistic infections, in addition to pain and discomfort. Therefore, it is highly desirable to develop agents and methods for using them to promote a rapid wound healing response.
Once a wound occurs, the body initiates a coordinated repair response which is a complex process of events involving both humoral and cellular elements, and which occurs over a time period of days to weeks. In particular, it has been shown that wound healing depends on the interactions between specific cell types, cytokines and extracellular matrix (Clark, 1989, Curr. Opinion Cell Biol. 1: 1000). A first step in wound healing involves the action of blood-borne cells known as platelets. These cells aggregate at wound sites and form a temporary barrier that prevents blood loss. Platelets achieve this function by secreting thrombin, which catalyzes blood clot formation, and other factors, which serve to attract other cells into the damaged area.
After the first 24 hours, additional cellular elements arrive and contribute to the wound healing process. Blood-borne neutrophils and monocytes migrate into the wound site. These cells function in part by neutralizing invading microorganisms and secreting enzymes that clear away the initial clot. During this second, often referred to as "inflammatory" phase of wound repair, macrophages play a primary role by secreting a variety of inflammatory cytokines such as tumor necrosis factor (TNF), the interleukins such as IL-1, IL-6, IL-8, transforming growth factor-.beta. (TGF-.beta.), etc., and growth factors such as epidermal growth factor (EGF), fibroblast growth factor (FGF) and platelet-derived growth factor (PDGF), that serve to combat infection and recruit additional cell types. These cell types include the epithelial and connective tissue cells particularly fibroblasts, that ultimately repair the site of tissue damage. The final phase of tissue repair is tissue remodeling, involving collagen cross-linking, collagenolysis and collagen synthesis for increasing structural integrity within the wound. Unfortunately, this entire process takes a relatively long time to complete.
Various approaches have been studied in recent years in an attempt to accelerate the wound repair process so as to prevent or minimize infections and further damage to the underlying tissues. A more traditional approach involves the grafting of healthy tissues upon a wound site, particularly the use of autologous tissues obtained from a different part of the body of the same individual (Bell et al. 1983, J. Invest. Dermatol. 81: 25). Another approach involves the administration of cytokines known to be capable of promoting chemotaxis and cellular proliferation. Such cytokines include PDGF, TGF.beta. and FGF (Pierce et al., 1989, J. Cell. Biol. 109: 429; Rappolee et al., 1988, Science 241: 708).